FIG. 1 depicts a conventional single-ended battery charging system 100. The charging system 100 generally includes a charger 102 and a battery pack 104, as is well known. The charger 102 may include a source 106 and a charger controller 108 to control the charging current from the source 106 to the battery 104. In the conventional charger architecture, for safety reasons, the controller 108 is typically configured to sense the battery temperature so that the controller can adjust the charging current based on the detected temperature, or perform Over-Temperature (OT) protection to stop charging the battery if the temperature exceeds a threshold. To that end, the battery pack 104 includes a thermistor, RTS, and the resistance of the thermistor varies with temperature. For a conventional negative temperature coefficient (NTC) or positive temperature coefficient (PTC) type thermistor, the resistance of RTS will change linearly as the temperature changes. The controller 108 may include a voltage sensing node VTS coupled to RTS to sense the change in voltage across RTS as a function of battery temperature. More particularly, the controller 108 may include a voltage divider circuit defined by RPU (a pull-up resistor) and RTS. A reference voltage, VREF, of the controller 108 may be used to drive the voltage divider circuit. In the idealized scenario, since VREF and RPU are fixed, as RTS changes due to temperature changes in the battery pack 104, VTS will change proportionally. Thus, the controller 108 can detect battery pack temperature information and, if necessary, adjust the charging current accordingly.
However, the temperature detection accuracy will be affected by the parasitic resistance of the power trace, depicted as RPAR. RPAR may result from, for example, the resistance of a PCB trace, power cord resistance, contact resistance from the connection between the battery pack 104 and the charger 102, or a combination of these. The voltage drop over RPAR will directly contribute to the sensing voltage over the thermistor RTS, and thus an error is introduced into the control of charging current based on temperature. In operation, the charging current delivered to the battery 104 will control the voltage drop over RPAR, which in turn will influence the detected voltage across RTS. In other words, VTS=VRTS+VRPAR, and so, for a given value of RTS (meaning the temperature is unchanged), as the charging current increases, VRPAR will increase and therefore VTS will increase, but not due to temperature changes in the battery pack. As a result of the influence of RPAR and assuming an NTC thermistor, the actual OT trigger point will need to be lower than the actual temperature of the battery, since RTS will drop lower to account for the parasitic resistance. This can introduce a significant safety concern about the operation of battery pack.